There is a long tradition of using neurophysiological techniques to aid in identifying the functional brain systems that subserve qualitatively different aspects of cognition. Likewise, deviance in neurophysiological responses to cognitive challenge has often been used to suggest regional abnormalities in clinical populations. Both types of effort depend in part on parametric assumptions about the relations of neurophysiological changes to extent of information processing. However, neurophysiological indices of cognitive effort or processing load for complex tasks have not been identified in electrophysiological research and have not been examined in work with cerebral blood flow or metabolism measures. This proposal will address this problem in four experiments. rCBF and EEG will be simultaneously assessed during task conditions which vary discretely in difficulty level. Experiment 1 and 2 will derive and cross-validate physiological features sensitive to difficulty levels and determine the extent to which intensity of manifestation and the spatial distribution of neural networks are altered by manipulation of cognitive effort. Experiment 4 will assess extent to which the derived features distinguish groups that differ in age and/or task-related abilities. These studies will test specific hypotheses concerning regional changes in neural activity that accompany variation in cognitive effort within individuals. It will also test the view that groups that differ in task-related abilities differ in the neurophysiological patterns, even when task processing load is titrated so that groups are equivalent in performance accuracy. Support of this hypothesis may raise serious concerns about standard comparisons of clinical and control groups in the neurophysiological concomitants of information processing.